The present invention relates to control apparatus and method for controlling certain processes including, for example, chemical processes or temperature related processes. Devices and methods within the scope of the present invention can be utilized as a primary means of control or can, advantageously, be utilized as supplemental control means for condition responsive control means commonly utilized in various processes.
Numerous prior art arrangements are known for controlling processes, for example both chemical and thermal, where a sensor means is provided to sense a variable condition of the process and the output from the sensor is fed back to a control means to modify operator devices to modify the variable, for example increasing flow, decreasing flow or other process variables. In such arrangements, the control is designed to provide modification in response to changes in the process variable in a selected range but a problem is encountered in the event of a rapid unanticipated change in the process variable under control, for example, a rapid increase or decrease in the process variable such as flow rate concentration or temperature.
Prior art systems can be modified and designed, to accomodate specified rates of change in such process variables but, the more sophisticated the controller system the more expensive the system. For example, differential controllers are known wherein correction is made in response to rate of change in the process variable where the magnitude of correction is determined by the time differential of the change in the process variable, that is the instantaneous rate of increase or decrease. Such devices are generally expensive and even where provided are subject to finite limitations.
Other applications are known where a normal condition exists but is subject to rapid change without advance notice and where correction is needed only in the event of such unexpected change. Such arrangements are commonly found in waste treatment process plants and other applications where certain minimal conditions, such as pH or other such parameters must be maintained and where the condition of the effluent stream is modified by addition of a second stream of selected pH to maintain the effluent stream above a given pH. However, in the event of an upset of a process or change in the condition of the second stream, the condition of the effluent falls below the prescribed minimum and rapid corrective action is required.